This applications is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-97416, filed on Mar. 29, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a cold cathode and a cold cathode discharge device.
2. Description of the Related Art
A cold cathode discharge device represented by a cold cathode discharge lamp has a simple structure, which does not employ any heating filaments. Accordingly, because the device can easily be miniaturized and operate at a low temperature, and moreover has a relatively long life, it is widely put to use recently for a wide variety of lighting or a backlight of a liquid crystal display device.
On the other hand, because the cold cathode discharge lamp keeps discharging by means of secondary electron emission from the cold cathode due to ion bombardment of the discharge gas filled therein, it is usually required that a bias voltage should be applied near the cathode portion in order to generate a very high electric field compared with a thermal cathode used for a thermal cathode fluorescent lamp, etc. While the lamp is turned on, it is therefore necessary to supply a high voltage thereto, and consequently the cold cathode discharge lamp is inferior to the thermal cathode discharge lamp with regard to the conversion efficiency of electric power into light, because of the high electric field and the high voltage mentioned above. However the cold cathode type lasts longer than the thermal cathode type, so that it is frequently used for some applications where exchanging the lamp is not easy. In consequence, it is desired that durability of the lamp and a discharge device should be much more improved.
Accordingly, the present invention is intended to solve the two problems i.e. light emitting efficiency and life of the cold cathode discharge device, and consequently to realize both higher efficiency of light emission and longer life of the device.
One aspect of the present invention is to obtain a cold cathode comprising a supporting member and an electron emitter supported by the supporting member and having an electron-emitting surface which emits electrons, the electron-emitting surface comprising a mixed phase of diamond phase and conductive carbon phase, and the conductive carbon extending in the form of a channel between the supporting member and the electron-emitting surface in the electron emitter. The cold cathode here means an electrode structure that does not have any heating filaments. Electron emission itself includes secondary electron emission due to ion bombardment, field emission, thermal electron emission caused by self-heat generation, and other emission according to a mechanism of such as thermionic field emission that is between thermal electron emission and field emission.
Furthermore, the diamond phase of the electron emitter preferably includes a donor impurity in this case.
Moreover it is desirable that the diamond phase should be constituted of a granular body, and a graphite or amorphous carbon layer should be formed on a boundary surface of the granular body.
The other aspect of the present invention is further intended to obtain a cold cathode discharge device comprising an envelope filled with a discharge gas therein and a cold cathode provided in the envelope, wherein the cold cathode comprises a supporting member and an electron emitter with an electron-emitting surface to emit electrons supported by the supporting member, the electron emitter comprising a mixed phase of diamond phase and conductive carbon phase, and the discharge gas containing a gas including an element with a principal radiation peak of 200 nanometers or less in wavelength.
Besides, it is desirable for the discharge gas to include xenon (Xe).
The essence of the present invention is to use a material of carbon system comprising diamond with a negative electron affinity or an electron affinity remarkably smaller than that of electrodes made of such as metal, and a granular boundary layer of graphite or amorphous carbon made of carbon that is the same as the diamond and having sp2 bonding, as an electrode for the cold cathode.
It also is desirable that the diamond should be high quality and has the Fermi-level raised by adding an impurity of donor nature, i.e. phosphor, sulfur, nitrogen, or alkaline metals thereto. Furthermore, the present invention is characterized in that electron-emission in the discharge lamp is preferably promoted by direct excitation due to excited radiation in the discharge lamp, by means of adding Xe, etc. having an excited radiation wavelength with an energy equal to or greater than the band gap of diamond, to the gas filled therein.
The cold cathode according to the present invention employs a carbon system electrode comprising diamond phase and graphite or amorphous carbon layer instead of a metallic electrode of such as nickel (Ni), which is used for conventional cold cathodes. Carbon is contained within the inside of the discharge tube together with mercury, so as to play a role to assist mercury to be excited effectively. Carbon is one of the elements that are the hardest of almost all elements with respect to spattering by argon (Ar) ion bombardment. On the other hand, life of the cold cathode discharge lamp finally depends on wear-off of the electrode by spattering, so that the carbon system material is very effective against such phenomenon.
However secondary electron emission is very small for the graphite system material. Because mechanism of electron emission for the cold cathode discharge lamp is secondary electron emission by ion bombardment of such as Ar filled therein (gamma action), the graphite system material was difficult to be applied actually as an electrode material for the cold cathode, although its anti-spattering property is superior.
The present invention makes efficiency of the secondary electron emission be consistent with the anti-spattering property by combining the diamond with the graphite system, resulting in realization of the cold cathode discharge device that has high efficiency and long life.